Well, you're PARTIALLY right - German thrash bands had a distinct aggressive edge to them, but saying that the so-called Teutonic Big 3 are all similar to each other is pushing it a bit if you ask me. Kreator got increasingly more melodic/rubbish, Destruction have pretty much been constant with their style (maybe with the exceptions of Release from Agony, Cracked Brain and Devolution - not to mention the dreadful Neo Destruction releases) and Sodom have probably switched their style up more than the other two combined.
Highly recommended listening though, here's a shortlist;
Kreator:
Pleasure to Kill, Terrible Certainty, Coma of Souls, Enemy of God
Destruction:
Pretty much anything, but I'd say their standouts are Infernal Overkill, Release from Agony, The Antichrist and Devolution. To make things easier, just get their album "Thrash Anthems" was was a bunch of re-recorded tracks from their first six releases.